1. Field of the Invention
The present invention relates to an image capturing apparatus and a control method therefor.
2. Description of the Related Art
Image capturing apparatuses such as video cameras include those that can change the frame rate when capturing images of subjects (hereinafter referred to as the “image-capturing frame rate”). Using such an image capturing apparatus, video data that has been captured and recorded at an image-capturing frame rate (e.g., 240 fps) higher than a normal image-capturing frame rate (e.g., 60 fps) can be reproduced at the normal frame rate. Such reproduction makes it possible to obtain slow playback video in which a subject motion is slower than actual motion.
The frame rate of a liquid crystal display of such an image capturing apparatus is generally equivalent to the normal image-capturing frame rate (e.g., 60 fps). Thus, in the case where the liquid crystal display is used as an electronic viewfinder (EVF) while video is being captured at a high image-capturing frame rate, the image capturing apparatus converts the frame rate of the video into the frame rate of the liquid crystal display by thinning out frame images for display.
Examples of the time when the image-capturing frame rate changes from “Normal” to “High Speed” include the following two cases. The first case is the time when video recording is started. For example, consider the case where the image capturing apparatus is set in the mode of performing recording at a high image-capturing frame rate. Even in such a case, before the start of the recording, the image capturing apparatus generally performs image capturing and display on the liquid crystal display at the normal frame rate for the purpose of, for example, reducing power consumption. For this reason, the image-capturing frame rate changes from “Normal” to “High Speed” at the time when recording is started. The second case is the time when the image-capturing frame rate is switched during video recording. For example, some image capturing apparatuses include an operation unit such as a “high-speed recording start button”. By pressing the “high-speed recording start button” in scenes for which slow playback is desired while video is being recorded at the normal image-capturing frame rate, a user can capture and record only some of the scenes at a high image-capturing frame rate.
Incidentally, in the case of video capturing, the exposure time of an image sensing element needs to be shorter than the interval between frames (which is the reciprocal of the image-capturing frame rate). For this reason, when the image-capturing frame rate changes from “Normal” to “High Speed”, video may be darkened as a result of the shortened exposure time. In view of this, a technique has been proposed in which the brightness of images is maintained at a predetermined level when the image-capturing frame rate has been changed (Japanese Patent Laid-Open No. 2007-195038). According to Japanese Patent Laid-Open No. 2007-195038, when the image capturing frame rate is increased, the aperture and the gain of image signals are adjusted so as to cancel out a change in the exposure time.
However, a certain amount of time is required for the aperture to be changed into the target state. For this reason, video is temporarily darkened and image quality decreases during the period until the aperture has completely changed. Furthermore, an increase in gain results in an increase in noise and degradation in image quality.